Experts observe a stable harbour seal population in the Wadden Sea

In August 2020, 28,352 harbour seals were counted in the Danish-Dutch-German Wadden Sea and Helgoland. This is the highest count yet recorded since 1975, although only with a slight increase of 2% compared to 2019. The number of newborns also reached a record high with 9,954 observed pups. In their new report “Trilateral surveys of Harbour Seals in the Wadden Sea and Helgoland in 2020”, the Expert Group on Seals (EG-Seals) of the Trilateral Wadden Sea Cooperation estimates that the total population of harbour seals in the Wadden Sea is 41,700, based on the assumption that that about one third of the seals were in the water during the survey. The trilaterally coordinated aerial surveys are conducted twice a year – during the pupping season in June and during the moulting season in August, when harbour seals spend more time on land.

The EG-Seals state that since 2012, they have been observing a stabilizing trend in the population, with an average annual growth rate of 1.2%, which is much lower than in previous decades. “Looking at the regions, we detect large increases and decreases from year to year”, says Ursula Siebert, chair of the EG-Seals and professor at the Institute for Terrestrial and Aquatic Wildlife Research (ITAW). “There could be different reasons for this, such as a variation in the survey date, varying numbers of seals hauling out, weather or a migration between areas. This is why it is important to treat the Wadden Sea as an ecological entity and coordinate the trilateral counts to get an overview on the status of the entire harbour seal population.”

In the Danish Wadden Sea, 2,256 seals were counted during the moult in August, which is 16% less than in 2019, while in Schleswig-Holstein’s Wadden Sea the number increased by 23% to 10,746 seals. In Lower Saxony and Hamburg, 7,553 seals were observed (14% lower than in 2019). In the Netherlands, the counts slightly increased by 4% to 7,661 individuals. On Helgoland, after a high regional growth of 33% in the past year, this year only 136 harbour seals were observed, which is 47% fewer harbour seals observed compared to 2019.

The overall number of newborn seals rose by 3% compared to the previous year to 9,954 pups, counted during the pupping season in June. This is the highest number registered yet and continues the series of record numbers observed in recent years. Divided into regions, in Denmark, where the count was extraordinarily high in 2019, 53% fewer pups were observed this year with a count of 429. In Schleswig-Holstein’s Wadden Sea, 4,499 newborns were observed (+21%). In Lower Saxony and Hamburg a moderate decrease of 8% was noted with 2,484 pups. In the Netherlands, 2,542 newborns were recorded (+9%). No pups were observed on Helgoland. The percentage of pups compared to the August moult count was 35%, the same as in 2019 and the second highest ever recorded.

After years of growth at 8.9% per year up to 2012, the population seems to have stabilised in the past 8 years growing at an average of 1.2% per year, and a population estimated at around 40.000 animals. The EG-Seals concludes that this trend is noteworthy as the pup production has increased in these 8 years to around 10.000 a year, but does not seem to contribute in the growth of the overall numbers. Therefore further studies are required to understand the underlying mechanisms.

Harbour seals, as well as grey seals, are one of the iconic species of the region. Part of the Trilateral Wadden Sea Cooperation, the trilateral Expert Group Seals (EG-Seals) (former TSEG) coordinates the counts and harmonizes the data from across the Wadden Sea region. The harbour seal is trilaterally protected under the Agreement on the Conservation of Seals in the Wadden Sea (WSSA) concluded under the auspices of the UN Convention on the Conservation of Migratory Species of Wild Animals (CMS). CWSS acts as the secretariat of WSSA, which will celebrate the 30th anniversary of its entry into force in October 2021.

The full 2020 report and former reports can be downloaded here.